Disrupting the CD40-CD40L pathway is an effective means to prolong allograft survival. However, the majority of studies have disrupted the pathway by targeting CD40L with anti-CD40L mAb or the use of CD40L-/- mice. Hence, the individual contributions of CD40 vs. CD40L to the rejection process have not been defined. The use of anti-CD40L mAb is frequently referred to as a blockade" of the CD40-CD40L 'pathway, implying a physical disruption of the interactions between these molecules. In fact, the precise mechanisms by which anti-CD40L mAb mediate their protective effects have not been established. Evidence suggests that anti-CD40L mAb may induce apoptosis of T cells or may result in complement-mediated cytolysis of T cells. Much less is known about the role of CD40 in the rejection response. The overall hypothesis of this proposal is that disrupting CD40-CD40L interactions by targeting CD40 vs. CD40L yields distinct immunological outcomes, which differentially impact long-term graft acceptance. Hence, the proposed studies will define individual roles for CD40 and CD40L in the mouse vascularized cardiac allograft model. Specifically, CD40-/- mice, agonistic anti-CD40 mAb, CD40L-/- mice, and anti-CD40L mAb will be employed in conjunction with donor-derived dendritic cells (DC) to elucidate the roles of CD40 and CD40L, and how DC bypass the need for this pathway in the rejection process. Specific Aim 1 will directly assess the roles of donor and recipient CD40 to rejection by combining agonistic anti-CD40 mAb with CD40-1- donors or recipients, CD40L-/- recipients, or recipient treatment with anti-CD40L mAb. The contributions of donor DC and recipient CD4+ and CD8+ cells will be investigated. Specific Aim 2 will address the mechanism of action of anti-CD40L mAb using Bcl-xL transgenic mice (T cells resistant to apoptosis), complement -/- mice (no complement mediated cytolysis), and CD40-/- mice (to investigate direct effects of anti-CD40L). Specific Aim 3 will test the hypothesis that distinct forms of regulation develop when CD40 vs. CD40L is targeted to promote long-term graft acceptance. Identifying the mechanisms by which CD40 and CD40L contribute to rejection should facilitate development of therapies that target this critical pathway in transplant rejection.